Some communications systems, and particularly a code-division multiple access (CDMA) digital cellular radiotelephone system, require that a fully functional base-station be synchronized within .+-.3 .mu.S of Global Positioning System (GPS) time. In the event of a GPS failure, .+-.10 .mu.S synchronization window is allowed. The present method of providing the required synchronization is through the use of the GPS satellite network and a GPS receiver. In the event of a GPS failure, a Rubidium oscillator is the present solution to providing redundancy. The use of the Rubidium oscillator for redundancy is a costly alternative for a limited amount of failure protection. A typical Rubidium oscillator will keep a previously synchronized base-station operational for a minimum of nineteen (19) hours, assuming the following: ##EQU1##
This extremely short time is obtained at an extremely high cost, approximately $4,000 per oscillator. To maintain the required synchronization between CDMA base stations, a central clocking source must be distributed to all base stations in the network. The use of a "Free Running" oscillator will allow a base-station to drift out of synchronization due to tolerances. The greatest difficulty in utilizing a central clocking source is the distribution of that signal with a consistent and predictable propagation delay throughout the network.
Thus a need exists for an apparatus and method that provides reliable redundancy for an extended period of time, while maintaining a cost-effective system design.